It is well known in the field of gas turbine engines to cast components from metals and alloys using a process known as directional solidification. Directional solidification involves the controlled cooling of molten material in a mould to create a unidirectional grain structure and often a single crystal structure in the finished component. The term “single crystal” as used herein is intended to include castings with low angle grain boundaries where the crystal structure is maintained throughout the component.
FIG. 1 illustrates apparatus known to be used to perform directional solidification casting. The apparatus comprises a pouring cup 1 into which molten material M is poured. The pouring cup 1 sits on a cylindrical support column 7 having a centreline C-C. A plurality of feed channels 2 extend radially around the centrally arranged cup 1 to a top end of the moulds 3. Molten material M poured into the cup 1 flows along the feed channels 2 and into the moulds 3. Each mould 3 is provided with a grain selector 4a at a bottom end which terminates in a starter block 4b. The starter blocks 4b sit on a chill plate 5 which is maintained generally at a temperature below the melting point of the material M creating a temperature gradient from the bottom to the top of the moulds 3. During the pouring process, the moulds 3 are enclosed by a heat source 6 which encircles the cup 1 and the array of moulds 3. The assembly is drawn in a controlled manner out of the heat source 6 in the direction of arrow A to ensure directional solidification from the bottom of the moulds 3 to the top of the moulds 3. The combination of the grain selector 4a and starter 4b with controlled cooling encourages growth of a single crystal structure in the solidifying casting.
It will be appreciated that this is just one representative example. Other known apparatus include arrangements where the pouring cup is not at the centre of the mould array, feed channels which feed to regions of the mould other than the top and alternative methods for controlled cooling may be used, for example liquid metal cooling.
In many applications it is desirable to have grain growth in a direction which is inclined to the direction (Arrow A in FIG. 1) of cooling In such cases, crystal growth in terms of dendritic growth during solidification will converge upon a wall of the mould or diverge from the wall in respective converging and diverging dispositions. As differently directed dendrites approach each other, a dominant growth will tend to snuff out an opposing growth. Thus, it is desired to ensure that growth in the preferred direction is enabled to be dominant.
The Applicants own prior published patent no EP1793020 (B1) is more particularly directed to controlling the direction of dendritic growth where the preferred direction is inclined to the direction of cooling. Using apparatus broadly similar to that described, a seed crystal having the required directional dendritic growth is located in a sacrificial portion of the mould and is partially melted. Molten material is then added to the melted portion of the seed crystal to fill the mould. Directional cooling is applied as previously discussed. The method of EP1793020 (B1) dispenses with the previously used grain selector of a “pig-tail” design using only a selectively positioned and proportioned cylindrical tube grain selector between the seed and main body of the mould. Dendrites following the growth pattern of the un-melted seed crystal are filtered from those growing in conflict with this primary grain to provide the desired dominant grain structure in the solidified casting.
Whilst the method of EP1793020 (B1) is well suited for the purpose of blocking secondary grain growth in directions which compete with the primary grain growth, a further challenge arises. Thermal strains experienced by the primary dendrites as they grow through the grain selector can result in clumps of dendrites within the primary grain growth bending away from the initial direction of growth. The resulting misorientation can be up to 15° from the initial direction. This is accentuated for smaller diameters of the cylindrical tubes, typically 5 mm. Since the primary orientation in such seeded assemblies is typically inclined 15° to the vertical blade stacking axis, the bent cluster of dendrites can in some cases become the dominant growth direction if they align with the direction of the heat flux (arrow A in FIG. 1). They can consequently outgrow the initial off-axial primary <001> grain resulting in the formation of undesirable grain boundaries in the body of the cast component. Dendritic bending can occur prominently at sharply angled features of a mould geometry.
Cast components often undergo subsequent manufacturing operations before becoming a finished component. The cavity of the mould may define a geometry of a finished component and an additional sacrificial geometry which may be used in one or more subsequent manufacturing operations. The sacrificial geometry is removed when no longer needed for subsequent manufacturing operations leaving the finished component geometry. The sacrificial geometry may serve multiple functions, for example, it may be positioned to allow the cast component to be held securely during subsequent manufacturing processes. It is common for the sacrificial geometry to be provided at a bottom end of the mould (that is the relatively cooler end at which solidification commences). The integrity of a cast component can be improved if the occurrence of secondary growth dominance is restricted to the sacrificial geometry.